Control of frame timing on handover

ABSTRACT

The uplink transmission timing from a mobile communications device is defined with reference to the downlink reception timing of signals from a particular reference cell. When that reference cell is removed from the active set, there is defined a virtual reference cell, the timing of which is defined with reference to one or more of the cells remaining in the active set, such that the timing of this new virtual reference cell corresponds to the timing of the previous reference cell. The timing of the uplink transmission from the user equipment are then defined with reference to the new virtual reference cell, in the conventional way. This has the advantage that, following a soft handover, it is not necessary to adjust the timing of uplink transmissions from the user equipment.

TECHNICAL FIELD OF THE INVENTION

This invention relates to a mobile communications system, and inparticular to a method of controlling the timing of frames defined fordata receipt and transmission in a mobile communications device. Otheraspects of the invention relate to the mobile communications deviceitself, and to the network which implements the method.

BACKGROUND OF THE INVENTION

In the proposed Universal Mobile Telephony System (UMTS), defined by thestandards published by the 3rd Generation Partnership Project (3GPP), amobile device, or user element UE, can establish links with severalcells in the cellular radio network.

The data received by the user equipment from each of the cells istransmitted in frames on respective downlink channels. The frames in therespective downlink channels are not synchronized with each other.Moreover, because of possible movement of the user equipment relative tothe base stations in the different cells, the timing of each downlinkframe, relative to the other received downlink frames, can change.

The user equipment transmits data in frames on uplink channels to thebase stations in the cells with which the user equipment has links.Thus, there is defined for the user equipment a transmission frametiming, which is common to all of the uplink channels. This transmissionframe timing is defined with reference to the downlink frame timing ofone of the active links.

Thus, in the Technical Specification 3GPP TS 25.133 v3.8.0, section 7,and the associated test case at section A.7, there is described a methodfor defining the timing of the, uplink transmission frame. Specifically,in the case where there is a single active link, the uplink frametransmission takes place a time T₀ (=1,024 chips) after the reception ofthe first detected path of the corresponding downlink frame.

When the user equipment has one active link with a cell, itstransmission frame timing is set with reference to the downlink frametiming of that cell, and the transmission frame timing is still definedwith reference to the downlink frame timing of that first cell, evenwhen a second cell is added to the active set. However, in that case,when the first cell is removed from the active set, the uplink transmittiming is adjusted, at a defined adjustment rate, until the uplink frametransmission takes place T₀ (=1,024 chips) after the reception of thefirst detected path of the downlink frame from the remaining activelink.

Thus, according to the published Technical Specification, there isdefined a reference cell, that is the cell with reference to which theuplink frame transmission is defined. When there are two active links,and the reference cell is removed from the active set, the remainingcell becomes the reference cell.

However, this does not indicate how the uplink frame transmission shouldbe defined in the event of a soft handover, when there are more than twoactive links, and the reference cell is removed from the active set.

SUMMARY OF THE INVENTION

According to the published Technical Specification, discussed above, theuplink transmission timing is defined with reference to the downlinkreception timing of signals from a particular reference cell. When thatreference cell is removed from the active set, the TechnicalSpecification envisages that another cell from the active set be definedas the new reference cell, and that the uplink transmission timingshould be adjusted so that it is defined with reference to the downlinkreception timing from the new reference cell.

However, large adjustments are undesirable, since they can have theeffect that another active cell must be removed from the active set,because its transmissions no longer fall within the receiver window ofthe user equipment. On the other hand, choosing a new reference cellsimply because it would require the smallest adjustment may also beundesirable because, if that cell were itself to be removed from theactive set, it would then be necessary to choose another new referencecell.

According to the present invention, therefore, there is defined avirtual reference cell, the timing of which is defined with reference toone or more of the cells remaining in the active set, such that thetiming of this new virtual reference cell corresponds to the timing ofthe previous reference cell. The timing of the uplink transmissions fromthe user equipment are then defined with reference to the new virtualreference cell, in the conventional way.

This has the advantage that, following a soft handover, it is notnecessary to adjust the timing of uplink transmissions from the userequipment.

The virtual reference cell may be defined with reference to the activecell which first joined the active set, the cell whose downlinktransmission timing most closely corresponds to the downlinktransmission timing of the previous reference cell, or the cell fromwhich the strongest signal is being received, or may be defined withreference to the average timing of all of the cells in the active set.

According to another aspect of the present invention, there is provideduser equipment which is able to define a virtual reference cell asdescribed above, following a soft handover.

It should be emphasised that the term “comprises/comprising” when usedin this specification is taken to specify the presence of statedfeatures, integers, steps or components but does not preclude thepresence or addition of one or more other features, integers, steps,components or groups thereof.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic representation of a part of a mobilecommunications network in accordance with an aspect of the invention.

FIG. 2 is a block schematic diagram of a mobile communications device inaccordance with an aspect of the invention.

FIG. 3 is a timing diagram showing the relationship between uplink anddownlink frame timing in accordance with the invention.

FIG. 4 is a flow chart illustrating a method in accordance with anaspect of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates a part of a mobile communications network inaccordance with the present invention. Specifically, FIG. 1 shows a userequipment (UE) 10, which is moveable within a network of base stationsystems (BSS) 12, 14, 16, 18. The base station systems (BSS) 12, 14, 16,18 have respective overlapping coverage areas, or cells. It will beobvious that the network contains more base station systems than thefour shown in FIG. 1. Each of the base station systems 12, 14, 16, 18has a respective connection (not shown in FIG. 1) to a datacommunications network.

The invention is described herein with reference to the Universal MobileTelephony Service (UMTS), as defined by the 3rd Generation PartnershipProject (3GPP).

In such a system, the user equipment 10 may have active radio links withmore than one base station system. The base station systems with whichthe user equipment has active radio links are defined as the active set.In use, for example as the user equipment moves further from or nearerto particular base station systems, some radio links may be lost, andnew radio links may be set up. This process of changing the membershipof the active set is known as soft handover.

FIG. 2 is a schematic diagram of the relevant parts of the userequipment 10. It will be apparent that the user equipment 10 has manyother features and functions which are not described herein, as they arenot essential for an understanding of the present invention. The userequipment 10 takes the form of portable radio communication equipment,which may for example be a mobile telephone, or personal digitalassistant (PDA) with wireless communication facility. The UE 10 has anantenna 22, which receives and transmits signals over the air interfaceto one or more of the base station systems in the network. Receivedsignals are passed to transceiver (TRX) circuitry 24, and then to aprocessor 26. Conversely, signals for transmission are passed from theprocessor 26 to the transceiver circuitry 24, and then to the antenna22.

The processor 26 is responsible for separating the wanted signals outfrom all of the received radio signals, and is also responsible forreceiving the data which is to be transmitted, and converting it into aform which is suitable for transmission over the air interface.

FIG. 3 is a timing diagram, in which FIGS. 3(a) and 3(b) show thesituation when the user equipment 10 has one active radio link. In thatcase, the user equipment sets the timing of a downlink frame 31, namelythe frame timing of the detected downlink signals on the active radiolink.

FIG. 3(b) shows the uplink frame transmission timing. As isconventional, the uplink frame 32 begins T₀ (=1,024 chips) after thereception of the first detected path of the corresponding downlink frame31.

In use, for example as the user equipment moves relative to the basestation with which it has the active radio link, it may be necessary toadjust the frame timings from one frame to the next.

The start of the downlink frame 31 can be referred to as the receivedradio link timing, and this also defines a receiver window, whichextends a fixed time (=148 chips) before and after the received radiolink timing. Other radio links are only permitted if their respectivereceived radio link timings are within this receiver window.

FIG. 3(c) shows the situation when the user equipment has three radiolinks. The signals received on the three downlink channels are receivedat different times, the respective downlink frame timings beingillustrated by the reference numerals 33, 34, 35. FIG. 3(c) shows therelative timings of the three windows at one particular instant.However, as before, movement of the user equipment relative to the basestations can mean that these relative timings change in use.

As described above with reference to FIG. 3(b), the transmit frame 36 isdefined with reference to one of the reception frames. In this case, thecell which has the downlink frame timing 33 is defined as the referencecell, and the transmitted frame 36 is set to start T₀ (=1,024 chips)after the start of the received frame 33, as shown in FIG. 3(d).

Also, the receiver window, within which the received frames of otheractive radio links must lie, is defined with reference to the start ofthe received frame 33.

FIG. 4 is a flow chart illustrating a method in accordance with theinvention, and FIGS. 3(e) and 3(f) show the relative frame timings whichare obtained. In step 41 of the process, it is determined within theuser equipment whether the reference cell has been removed from theactive set for that user equipment. If is has not, there is no change,and the process returns to the start. However, if the reference cell hasbeen removed from the active set then, in accordance with the presentinvention, a new virtual reference cell is defined.

Thus, in order to avoid the need to select a new reference cell from theremaining cells of the active set, and therefore the need to adjust thetransmission frame timing and the timing of the receiver window, avirtual reference cell is defined. The virtual reference cell is a cellwhich is not used for reception or transmission of signals over the airinterface, but which is used in determining the timing of transmissions.Specifically, as shown in FIG. 3(e), the virtual reference cell has aframe timing 37 which corresponds exactly to the receiver frame timing33 of the previous reference cell.

The timing of the virtual reference cell receiver frame 37 is definedwith reference to one of the remaining cells. In the case illustrated inFIG. 3(e), the receiver frame timing 37 of the virtual reference cell isdefined as being a time X after the receiver frame timing 35 of one ofthe remaining cells. This remaining cell could be chosen because it hasthe strongest signal, or because it is the earliest received signal, orbecause it is the cell which is closest in time to the originalreference cell. As an alternative, the timing of the new virtualreference cell could be determined relative to the average timing of theremaining cells.

Then, as shown in FIG. 3(f), the transmit frame 36 is defined withreference to the virtual reference cell reception frame 37. Because thevirtual reference cell is chosen to have a timing which is the same asthe previous reference cell, it is not necessary to make largeadjustments to the timing of the transmission frame 36. This means thatthe transmission frame timing does not need to be adjusted, andtherefore that the quality of the modulated signal received in the basestation systems will be improved. It further means that the receiverwindow does not need to be adjusted, and therefore that there is no riskthat useful radio links need to be removed from the active set becausethey no longer fall within the receiver window.

Of course, it should be noted that it remains possible to makeadjustments to the frame timings, for example because of frequencyerrors between the base station and the user equipment, or in order tocompensate for movement of the user equipment relative to the basestations, but these adjustments are relatively small.

Once a virtual reference cell has been defined then, in step 43 of theprocess shown in FIG. 4, it is tested whether the remaining cell, namelythe cell with reference to which the virtual reference cell is defined,has been removed from the active set. Provided that the remaining cellremains in the active set, no action is necessary. However, in the eventthat that remaining cell is removed from the active set, it is necessaryto redefine the timing of the virtual reference cell, for example withreference to one of the other remaining cells, or with reference to theaverage timing of all of the remaining cells.

There is therefore described a system for defining a transmission frametiming in the event of a soft handover, without interrupting existingtransmissions.

As described above, the system is used when the soft handover removes acell from the active set and a plurality of cells remain in the activeset. However, the system in accordance with the invention can also beused by defining a virtual reference cell when one or more cells areremoved from the active set and only one cell remains in the active set.

1. A method of defining an uplink transmission frame timing, for use ina mobile communications system in which a user equipment may have radiolinks with a plurality of cells, and in which the cells with which theuser equipment has radio links define an active set, wherein the uplinktransmission frame timing is defined with reference to the downlinktransmission frame timing of a reference cell selected from said activeset, the method comprising: when the reference cell is removed from theactive set, defining a virtual reference cell, the timing of which isdefined with reference to one or more of the cells remaining in theactive set, such that the timing of the virtual reference cellcorresponds to the timing of the previous reference cell; and definingthe uplink transmission frame timing relative to the timing of thevirtual reference cell.
 2. A method as claimed in claim 1, wherein theuplink transmission frame timing is defined to be a fixed time after thevirtual reference cell timing.
 3. A method as claimed in claim 2, foruse in UMTS, wherein the uplink transmission frame timing is defined tobe T₀ (=1,024 chips) after the virtual reference cell timing.
 4. Amethod as claimed in claim 1, wherein the virtual reference cell timingis defined with reference to the active cell which first joined theactive set.
 5. A method as claimed in claim 1, wherein the virtualreference cell timing is defined with reference to the active cell whosedownlink transmission timing most closely corresponds to the downlinktransmission timing of the previous reference cell.
 6. A method asclaimed in claim 1, wherein the virtual reference cell timing is definedwith reference to the active cell from which the strongest signal isbeing received.
 7. A method as claimed in claim 1, wherein the virtualreference cell timing is defined with reference to the average timing ofall of the cells in the active set.
 8. A mobile communications device,for use in a mobile communications system in which a mobilecommunications device may have radio links with a plurality of cells,and in which the cells with which the device has radio links define anactive set, wherein the device comprises means for defining an uplinktransmission frame timing with reference to the downlink transmissionframe timing of a reference cell selected from said active set, andwherein the device is adapted, when the reference cell is removed fromthe active set, to: define a virtual reference cell, the timing of whichis defined with reference to one or more of the cells remaining in theactive set, such that the timing of the virtual reference cellcorresponds to the timing of the previous reference cell; and to definethe uplink transmission frame timing relative to the timing of thevirtual reference cell.
 9. A mobile communications device as claimed inclaim 8, wherein the uplink transmission frame timing is defined to be afixed time after the virtual reference cell timing.
 10. A mobilecommunications device as claimed in claim 8, for use in UMTS, whereinthe uplink transmission frame timing is defined to be T₀ (=1,024 chips)after the virtual reference cell timing.
 11. A mobile communicationsdevice as claimed in claim 8, wherein the virtual reference cell timingis defined with reference to the active cell which first joined theactive set.
 12. A mobile communications device as claimed in claim 8,wherein the virtual reference cell timing is defined with reference tothe active cell whose downlink transmission timing most closelycorresponds to the downlink transmission timing of the previousreference cell.
 13. A mobile communications device as claimed in claim8, wherein the virtual reference cell timing is defined with referenceto the active cell from which the strongest signal is being received.14. A mobile communications device as claimed in claim 8, wherein thevirtual reference cell timing is defined with reference to the averagetiming of all of the cells in the active set.